This invention relates to foams. In one aspect, this invention relates to crosslinked polyolefinic foams, particularly crosslinked ethylenic polymer foam structures, while in another aspect, the invention relates to a process for making such foams. In yet another aspect, the invention relates to a dual cure process for making crosslinked polyolefinic foams with enhanced physical properties.
Crosslinked foams, e.g. crosslinked polyolefinic foams, are used in a variety of applications where cushioning under high or dynamic loading is needed. These foams are usually manufactured using a chemical blowing agent, e.g. azodicarbonamide, in combination with crosslinking induced by peroxide decomposition or electron beam irradiation. When exposed to elevated temperature (&gt;130.degree. C.), the blowing agent decomposes into a gas, e.g. nitrogen, and the polyolefinic matrix is crosslinked simultaneously via peroxide decomposition. By achieving an optimum level of tensile properties at elevated temperatures by crosslinking, the decomposed gas is allowed to expand controllably to produce foams with desirable cell sizes. Crosslinking and blowing of the foam may be performed either sequentially or simultaneously. Very small cell size (about 100 .mu.m diameter) foams are produced via simultaneous crosslinking and blowing agent decomposition under pressure as, for example, in compression or injection molding at elevated temperature. Very small cell size foams yield maximum cushioning from dynamic or high loading. Other foaming methods, e.g., extrusion, do not yield as small an average foam cell size and are not suitable for use in demanding cushioning applications such as foamed midsoles for athletic shoes, carpet underlay, cushioned vinyl flooring, gaskets, and the like. Foamed midsoles, in particular, require excellent creep and fatigue resistance, and hysteresis properties.
Several methods are known for crosslinking polyolefinic materials. Some common methods include the use of free radicals (e.g. peroxide and electron beam irradiation), sulfur cure, and moisture cure methods (e.g. those using silane grafted polyolefins or chlorosulphonated polyethylene). Crosslinking the polyolefinic matrix stabilizes the foam expansion process by increasing the extensional viscosity (melt strength) of the polymer and minimizing cell wall collapse. Furthermore, crosslinking enhances the physical properties (e.g. tensile strength, elastic recovery, creep, etc.) of the foamed article by establishing a molecular network within the polymer matrix. Higher levels of crosslinking result in higher tensile, elastic recovery, and creep properties. Fully crosslinked (100% gel as measured by ASTM D-2765-84) polyolefin-based foam structures would exhibit maximum tensile strength, elastic recovery and creep and fatigue resistance properties.
The crosslinking level, however, is typically restricted to 50-70% gel for optimum foam expansion. Optimally crosslinked foams utilize a sufficient amount of blowing agent to produce a foam to its minimum possible density without cellular collapse. Excessively crosslinked polyolefins restrict foam expansion during processing, yielding higher than desirable foam densities. As such, an upper crosslink or gel limit exists during the foam expansion process. Polyolefin-based foam compounds that have been crosslinked to 100% gel will not expand due to the very high elevated temperature tensile strength of the compound which restricts gas bubble expansion.
One potential method to further enhance the tensile, elastic recovery, and creep properties of the foamed article is post-expansion foam curing. Curing after foam expansion is not practical with peroxide or sulfur due to their consumption during the expansion process. Excessive peroxide or sulfur levels result in an excessive level of crosslinking which, in turn, interferes with the expansion process for the reasons already described. Mixed peroxides with different half-lives are also a possibility to further crosslink expanded foam, but this requires high oven temperatures to initiate the second-in-time peroxide. Such a process has limited utility in several finished article applications. Electron beam irradiation can also be used to further cure the foam but due to the high free surface-to-volume ratio of the foamed article, the polyolefinic foam oxidatively degrades when exposed to electron beam radiation.
One application, where superior fatigue and creep resistance properties are critical is a foamed midsole for an athletic shoe. Foamed midsoles provide a cushioning or impact absorbing characteristic to an athletic shoe. The typical useful life of a cushioning midsole is approximately 500 miles of running. With repeated wear of the shoe, the foamed midsole breaks down (the foam cells collapse) and the ability of the midsole to provide cushioning diminishes. The shoe in this condition is called a "dead" shoe. Wearing a "dead" shoe during high impact athletic activities such as running, walking or an aerobic workout can cause serious injury to ankle, knee, back or other parts of the body. Accordingly, athletic shoe manufacturers have a continuing interest in technology that can extend the useful life and cushioning ability of a midsole. Creep and fatigue resistance properties of a foamed midsole can predict the useful life of a shoe.
Another application where creep and fatigue resistance properties are critical is in cushioned vinyl flooring. Cushioned vinyl refers to the thin layer of foam on the underside of vinyl flooring. For this application, dynamic fatigue and static creep resistance are critical properties for enhanced performance and durability of the cushioned vinyl flooring.